The invention relates to an electron gun.
In numerous electronic devices and tubes, the electron beam produced by the gun is subjected, in the first part of its travel, to the action of means which impart thereto a convergence towards a point situated at a greater or lesser distance in front of the cathode. Beyond, different treatments are applied thereto, related to the use for which the unit is intended in which is incorporated the gun by which it is produced. These treatments are outside the scope of the invention.
Ideally, the point of convergence, where the electron paths cross, is a point in the geometrical sense of the word; in fact, considering the mutual repulsion of the charges which form it, the beam still presents at the point of convergence, and whatever the means used, a certain section whose area depends on numerous parameters and characteristics of the gun.
The electrons, after passing through this zone of minimum section, known also under the name of "cross-over", will form an image of this section on the part of the tube intended to receive their impact, screen, target . . .
The diameter of this impact depends on that of the cross-over in question; it is then desirable to reduce it to the maximum so as to improve the resolution of the surface subjected to the impact and, for this, to reduce the dimension of the cross-over.
This approach is however limited by the above-mentioned repulsion, i.e. the space charge within the beam.